1. Field of the Invention
One aspect of the present invention relates generally to an excavator for breaking-up hard soils, rock, or concrete into manageable sized pieces for subsequent handling or processing. The excavator acts on an existing ground surface, acting on a layer of material to define a new ground surface that is below the original. The process is used for road construction and mining. This aspect of the present invention relates more particularly the apparatus, which allows control of the depth of cut and of the orientation of the resulting new ground surface.
2. Description of the Related Art
Road Bed Preparation
In the preparation of a road bed one critical function is to establish the proper lateral grade. In most cases the desired lateral grade is level, with the exception of regions where the road curves and a banking effect is desirable. In both cases, when constructing new roads the grade of the native topography will typically need to be modified to achieve the desired grade. Certain ground conditions prohibit excavation in a manner wherein very fine adjustments can be made. These include conditions of rock and very hard soils. In these conditions the surface is typically excavated below the desired level, and finer more manageable materials backfilled to bring the grade to the desired level.
The process of replacing a damaged road surface often begins with the step of removing the existing road surface. The current methods of removing existing road surfaces of concrete are complicated by the existence of steel reinforcing rod that is integral to the concrete road surface. Current techniques of breaking up the road surfaces are slow and labor intensive often including the use of some form of impact wherein the existing road surface is struck from the above and broken into smaller pieces, and at the same time separating the reinforcing rod.
Mining
Many types of non-metallic rock are mined from shallow open-pit mines called quarries. The process is known as quarrying, open cast or surface mining. One quarrying technique involves drilling and blasting to break the rock. When usable rock is found, the surface is cleared to expose the desired rock. The area being mined is then drilled and blasted, a large number of low-powered explosives detonated at the same time to shatter the rock. The drillings are controlled to a depth to stay within the strata of desirable rock, as may have been determined by preliminary exploratory drillings. A single blast produces as much as 20,000 tons of broken stone. The broken stone is then loaded by handling equipment and transported to additional equipment to be crushed into smaller pieces and separated into uniform classes by screening methods. During that time the broken stone is exposed to the elements and some may be affected by weathering damage. This process is relatively labor intensive, produces work-in-process subject to damage. New techniques are recently being developed.
One such technique of quarrying is labeled as percussive mining in U.S. Pat. No. 5,338,102. In this reference a percussive mining machine is utilized to successively strike or impact the material with a cutting tool. In this case the cutting tools are mounted to a rotating drum that is propelled on a mining machine. The mining machine illustrated includes components representative of many machines which have recently been developed for this application. The machines typically include some form of ground drive, supporting frame for the drum, power unit to provide power to rotate the drum, a conveyance mechanism and some form of height control, to control the position of the drum. Examples of other machines, built specifically for this application, can be found in U.S. Pat. Nos. 5,092,659; 5,577,808; and 5,730,501. These machines are highly specialized, with limited additional use.
An example of a more versatile machine, built on a more generic platform, can be found in U.S. Pat. No. 4,755,001. This reference discloses an excavating machine that consists of a digging head mounted to an elongated digging member, both mounted to a main frame. The main frame resembles machines currently known as track trenchers.
Track trenchers, as is illustrated in FIG. 1, were originally designed for forming trenches for the installation of drainage lines or other utilities in open trench installations. The basic components of a Track Trencher 10 include:    1) a main frame 30,    2) a set of ground engaging track assemblies 20 which are fixedly supported by the main frame 30 in a manner that allows the drive sprocket 22 to be driven to propel the machine along the ground,    3) a power unit 40 typically a diesel engine, and    4) an excavation boom assembly 50 which is relatively narrow, as compared to its length, as most trenches are much deeper than they are wide.
The power unit 40 provides power to the driven/drive components of the machine.
This is typically comprised of a diesel engine and a hydraulic system. The hydraulic power is transferred to various actuators mounted on the machine to perform the desired operations including:
                1) a hydraulic motor 24 mounted onto the track drive frame that drives the track drive sprockets 22,        2) a hydraulic motor 52 mounted on frame 30 that supports and drives a sprocket which drives the excavation chain 54 that is supported on an idler sprocket 56 which is supported by the boom frame 51, and        3) a hydraulic system that includes cylinders 62 to raise and lower the excavation assembly.        
In trenching the primary parameter that needs to be controlled is the depth of the trench. The machine provides this control by controlling the position of the boom relative to the ground engaging tracks, typically allowing the boom to pivot around an axis defined by the machine frame. This pivot is designed robustly to handle the severe loading, particularly experienced when excavating rock. Typically the only movement of the boom relative to the frame is provided by pivoting about this axis.
Controlling the height of each ground drive unit, track, independently allows the frame to be kept level and thus the orientation of the resulting trench can also be controlled. However, this technique of orientation is not ideal in that the entire machine is being controlled resulting in higher power requirements and reduced responsiveness.